Reverse switching rectifier and method for making same

ABSTRACT

A reverse switching rectifier is described in which a PNPN semiconductor structure has a specially adapted N-type end zone or cathode-emitter zone. The N-type end zone penetrates to two different levels in the semiconductor body. A deep central portion and a shallow peripheral portion of the N-type end zone are produced by etching a cavity in the center of the body followed by diffusion of N-type dopant material. The exposed surfaces of the N-type end zone are then metallized to provide electrical and thermal contact thereto.

GOVERNMENT CONTRACT

This invention was made in the course of or under United States AirForce Contract F29601-74-C-0021.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to semiconductor switching devices andmore particularly to two terminal thyristor devices or reverse switchingrectifier devices, hereinafter designated RSR devices.

2. Description of the Prior Art

RSR devices of the prior art have a general structural configurationshown in FIG. 1 wherein a body of semiconductor material in the form ofa wafer 10 is doped to provide four alternate semiconductivity zones. Anend zone 12 of P-type semiconductivity extends from one major surface 11of the wafer 10 into the semiconductor material to meet a middle zone 14of N-type semiconductivity. PN junction 13 is formed at the interface ofzones 12 and 14. Similarly, P-type middle zone 16 forms PN junction 15with zone 14. Zone 16 extends from PN junction 15 to end zone 18 ofN-type semiconductivity located in an inner portion of the wafer 10where PN junction 17 is formed. In addition, zone 16 typically extendspast zone 18 to the outer portion of major surface 21. Zone 16 serves asthe base of the device, and zone 18 serves as the emitter. Typically, ashorted emitter construction is used whereby a cathode electrode 22 isaffixed to major surface 21 contacting the emitter zone 18 and aperipheral portion of the base zone 16 surrounding the emitter zone 18.The electrode 22 may be provided, for example, by aluminum deposition ina known manner. A supporting anode electrode 24 is affixed to majorsurface 11 to provide good electrical and thermal contact to zone 12 aswell as to provide mechanical support for the wafer 10. Typical examplesof metals used for the electrode 24 are molybdenum and tungsten, whichare preferred for their favorable expansion properties. The wafer 10 hasa beveled edge 25 produced in a known manner in order to optimizeelectrical characteristics. Disposed on the beveled edge 25 is aninsulating and protective coating 26. The coating composition and mannerof application is known in the art, a high temperature curing siliconevarnish being an example of a suitable coating material.

The RSR device of the prior art shown in FIG. 1 operates as anelectrical current switch. Briefly described, the RSR blocks voltage inboth directions unless the device is turned on in which case it carriescurrent in the forward direction as indicated by the arrow 27. The RSRdevice may be turned on in the presence of a forward voltage, asindicated by the polarity marks + and -, by impressing a forward voltagepulse across electrodes 24 and 22, which pulse has a sufficiently highDV/DT to cause the device to turn on.

It has been found that device structures of the prior art, as shown inFIG. 1, do not turn on uniformly along PN junction 15, rather such priorart devices initially turn on in a relatively small region located underan edge of the emitter zone 18 causing hot spotting and failure of thedevice. An example of a typical failure mode is illustrated in FIG. 1 inwhich emission of electrons from zone 18 into zone 16 more readilyoccurs in the dashed region 28 causing initial conduction of currentthrough PN junction 15 to pass through the relatively small area ofregion 28 as illustrated by path 29. The very high current density alongpath 29 causes localized heating which permanently destroys the blockingcapability of PN junction 15.

The present invention provides an RSR device which turns on uniformlythus eliminating the above-mentioned failure mode of prior art devices.

SUMMARY OF THE INVENTION

The present invention is a multi-layer semiconductor switching devicecomprising a body of semiconductor material and contacting means formaking thermal and electrical contact to said body; said contactingmeans comprising a metal cathode electrode; said body comprising anemitter zone of a first type of semiconductivity disposed along asurface portion in contact with said metal cathode electrode, a basezone of a second type of semiconductivity disposed in said body adjacentto said emitter zone and forming a PN junction with said emitter zone,said emitter zone having a central portion and a peripheral portionsurrounding said central portion, said peripheral portion extending intosaid body from said metal cathode electrode to a first level of said PNjunction, said central portion extending into said body from said metalcathode electrode to a second level of said PN junction, said secondlevel lying beyond said first level.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a vertical cross-sectional view of a device of the prior art;

FIGS. 2-8 are vertical cross-sectional views of a device of the presentinvention at various stages in the manufacturing process;

FIG. 9 is a greatly enlarged cross-sectional view of a portion of thedevice of FIG. 8;

FIG. 10 is a greatly enlarged paln view of the semiconductor surfaceillustrating the relative positioning of shunts; and

FIG. 11 is a vertical cross-sectional view of another device embodiment.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

As illustrated by FIG. 2, a body of semiconductor material in the formof a wafer 110 is prepared with three layers of alternatesemiconductivity type in a known manner. In a presently preferredembodiment, N-type silicon bar stock is sliced into wafers, each waferthen being subjected to P-type diffusion to produce a PNP waferstructure. For example, a boron, aluminum or gallium diffusion processmay be used to produce outer zones 112 and 116 of P-typesemiconductivity while inner zone 114 remains N-type semiconductivity.PN junctions 113 and 115 are thereby produced at the interfaces of thezones as shown. While not shown in FIG. 2, in actual practice, the boronor other P-type diffusant will penetrate the edge of the wafer 110 whichnecessitates subsequent processing to produce the desired structure.

FIG. 3 shows the next step in the manufacturing process wherein thewafer 110 is subjected to an oxidizing ambient to produce a silicondioxide layer 130 on all exposed surfaces.

In FIG. 4, the structure is shown after an opening has been etchedthrough the silicon dioxide layer 130 in a known manner which producesan exposed portion 129 and a covered portion 131 which comprise one ofthe two major surfaces of the wafer 110. In the presently preferredembodiment, circular geometries are employed due to the availability ofvarious diameters of cylindrical-shaped silicon bar stock. Silicon barstock is presently commercially available in various diameters up to amaximum of three inches. It is convenient to slice silicon bar stockinto circular-shaped wafers and to dope the wafers in circular-shapedgeometries. The present invention is not, however, limited to suchcircular geometries. Rather, these teachings may be readily applied tovarious other geometries and doping patterns.

FIG. 5 illustrates the wafer 110 after surface 129 has been etched toproduce a cavity or recess defined by cylindrical-shaped wall 133 andcircular-shaped surface 135. A preferred depth of recessed surface 135is about 5 microns, but a range of depths from 1 to 10 microns producesatisfactory results.

Next, the opening in the silicon dioxide layer 130 is increased indiameter as shown in FIG. 6 to expose an inner ring-shaped portion 131Aof major surface 131 while outer surface portion 131B remains covered bylayer 130.

Next, an N-type diffusion is performed which produces a two-levelemitter zone 138 of N-type semiconductivity as shown in FIG. 7. Apresently preferred dopant is phosphorus, for which gaseous diffusionmethods are well known. N-type emitter zone 138 forms PN junction 145with P-type base zone 116. A peripheral portion 138A of zone 138 lyingbelow surface 131A produces a first level of PN junction 145 designatedby numeral 145A. A central portion 138B of zone 138 lying below surface135 produces a second level of PN junction 145 designated by numeral145B. First level 145A lies on the order of about 1 micron deeper thanthe depth of recessed surface 135. The distance between second level145B and PN junction 115 is on the order of about 25 microns whichvaries somewhat in accordance with the desired electricalcharacteristics of the particular RSR device. Zone 138 has a preferredN-type dopant concentration on the order of 1 × 10²⁰ atoms/cm³ alongsurfaces 131A, 133 and 135. Zone 116 has a preferred P-type dopantconcentration on the order of 1 × 10¹⁷ atoms/cm³ along major surface131A- 131B. The N-type and P-type concentrations of zones 138 and 116are equal along PN junction 145; the concentration being on the order of4 × 10¹⁶ atoms/cm³ at first level 145A and 6 × 10¹⁵ atoms/cm³ at secondlevel 145B. Presently preferred device characteristics may be achievedwith doping concentration ranges from about 8 × 10¹⁵ to about 8 × 10¹⁶at first level 145A and from about 3 × 10¹⁵ to 8 × 10¹⁵ at second level145B.

FIG. 8 depicts a device of the present invention at the final stage inthe manufacturing process wherein the silicon dioxide has been strippedfrom the wafer 110 and the edges of the wafer have been beveled in aknown manner. Preferably at this stage, the surface area of majorsurface area of major surface portion 131A is about equal to that ofmajor surface portion 131B. Additionally, it is preferred that thecombined surface area of major surface 131A-131B is about equal to thesurface area of recessed surface 135.

A supporting electrode 152 is fused to flat circular-shaped majorsurface 153 of the wafer 110 in a known manner thereby making goodelectrical and thermal contact to P-type zone 112. A metal cathodeelectrode 154, preferably aluminum, is deposited on the wafer 110 tomake good electrical and thermal contact to N-type emitter zone 138 atsurfaces 131A, 133 and 135. Electrode 154 likewise contacts a surfaceportion of P-type base zone 116 surrounding emitter zone 138 anddesignated by numeral 131B.

By virtue of the electrical contact made at major surface 131A-131B, thePN junction 145 is shorted, thus providing the so-called "shortedemitter" arrangement. In addition, the P-type base zone 116 is alsoprovided with shunts or shorts as described below and illustrated inFIGS. 9 and 10, which shunts pass through N-type emitter zone 138 to theelectrode 154. Masking techniques for producing shunts are known in theart. Finally, an insulating protective coating 156 is applied to bevelededge 157 in like manner to the application of coating 26 of FIG. 1.

Test results show that the RSR structure of FIG. 8 will turn on in auniform manner thus eliminating the hot spotting which caused prior artdevices to fail. Such uniform turn-on is easily verified by use of aninfrared radiation detection apparatus.

A more detailed explanation of the operation of the present inventionwill now be given with the aid of FIGS. 9 and 10. FIG. 9 showscylindrical-shaped shunts 160 of P-type semiconductivity which passthrough zone 138 and make contact with electrode 154 at recessed surface135 (and at major surface portion 131A not shown in FIG. 9). The shunts160 are produced by masking surfaces 131A and 135 with silicon dioxedeprior to the diffusion step of FIG. 7.

FIG. 10 illustrates a plan view of a preferred shunt pattern showingsurface 135 after the N-type diffusion of zone 138. Each shunt 160 hasfour nearest neighbors, a preferred center-center separation distance Sbeing 20 mils, and a preferred diameter D being 5 mils. Satisfactoryresults may be achieved with S in the range from about 10 to about 35mils and D in the range from about 3 to about 11 mils with largerdiameters requiring a corresponding larger separation distance.

Referring again to FIG. 9, a forward bias voltage, applied to the devicein the polarity shown, reverse biases PN junction 115 which blocks allbut a small leakage current shown schematically by the arrows. Theleakage current flows to the cathode electrode 154 through the shunts160 which current creates a voltage differential between P-type basezone 116 and N-type emitter zone 138. As the leakage current increaseswith increasing forward bias, the base-emitter voltage eventuallyreaches a critical voltage, which for silicon devices is between about0.5 volts and 0.6 volts. When this critical voltage is exceeded, theemitter zone 138 begins emitting electrons into the base zone 116 whichtraverse PN junction 115, causing the device to turn on or fire in amanner known to those skilled in the art. In order to fire the RSRdevice at a predetermined time, a high dv/dt control pulse is impressedacross the device in the forward direction. Since the leakage currentpasses through each shunt 160, the critical voltage to fire is reachedsimultaneously across the area of the emitter zone 138. Therefore,initial conduction of current occurs uniformly through the emitter zone138.

Referring again to FIG. 8, edge firing is inhibited since the impedancepath through peripheral portion 138A of zone 138 is higher than theimpedance path through central portion 138B of zone 138. It is knownthat impedance is proportional to the ratio of dopant concentrations.The impedance from the base zone 116 to the cathode electrode 154 islowest through central portion 138B since the ration of N-type to P-typedopants is highest along surface 135. As previously discussed, theN-type concentration is the same along surfaces 131A, 133 and 135 whilethe P-type concentration is highest at major surface 131A-131B and dropsoff to a lower value at recessed surface 135 which drop-off is thenatural result of the diffusion process. The above differences inconcentrations are produced by etching away the high surfaceconcentrations of P-type diffusant in the step of FIG. 5.

The favorable N to P ratio in the central portion 138B of emitter zone138, combined with the multiplicity of shunts 160, produces an RSRdevice which fires uniformly. It will be apparent to those skilled inthe art that the presently preferred embodiment of the present inventionachieves a novel advance in the state of the art of RSR devices whichovercomes the edge firing failure mode of prior art devices. It will befurther apparent that a complementary device may be produced byinterchanging the P and N regions of the above-described device.

While the above described embodiment is the best mode presentlycontemplated by the inventor of carrying out his invention, an alternateembodiment is described below in conjunction with FIG. 11.

In comparing FIG. 11 with FIG. 8, it will be seen that the alternate ofFIG. 11 differs only in the shape of the emitter zone and associatedposition of the cathode electrode. The emitter zone 238 of FIG. 11 is atwo level zone consisting of deep central portion 238B and shallowperipheral portion 238A, with portions 238A and 238B both terminating atmajor surface 231. Cathode electrode 254 makes contact with base zone216 and emitter zone 238 at major surface 231 as shown. The device ofFIG. 11 is similar to the device of FIG. 8 in all other respects,similar numerals depicting similar portions of the devices.

In operation, the device of FIG. 11 does not have the highly favorable Nto P ratio in the central portion of the emitter zone as achieved atsurface 135 of the device of FIG. 8; however, both the devices of FIG. 8and FIG. 11 share the feature of a shallow peripheral portion of theemitter zone. Shallow peripheral portions 138A of FIG. 8 and 238A ofFIG. 11 tend to inhibit the edge firing failure mode of the prior artdevice shown in FIG. 1. It has been found that both the deviceembodiments of FIGS. 8 and 11 are significantly more reliable than theprior art device of FIG. 1, with the embodiment of FIG. 8 being mostsuperior.

In order to produce the embodiment of FIG. 11, a method similar to themethod of producing the embodiment of FIG. 8 is employed. The steps ofFIGS. 2, 3 and 4 are identical, but the step of FIG. 5 is eliminated, adiffusion step forming central portion 238B being performed instead.Next, the opening in the silicon dioxide is widened in similar fashionto the step of FIG. 6. A second diffusion step is then performed whichproduces peripheral portion 238A of two level emitter zone 238 of FIG.11. The remaining steps are substantially the same as those whichproduce the device embodiment of FIG. 8.

What is claimed is:
 1. A two-terminal multilayer semiconductor switchingdevice comprising a body of semiconductor material and contacting meansfor making thermal and electrical contact to said body; said contactingmeans comprising a metal cathode electrode; said body comprising anemitter zone of a first type of semiconductivity disposed along asurface portion in contact with said metal cathode electrode, a basezone of a second type of semiconductivity disposed in said body adjacentto said emitter zone and forming a PN junction with said emitter zone,said metal cathode electrode extending to contact said base zone, saidemitter zone having a central portion and peripheral portion surroundingsaid central portion, said peripheral portion extending into said bodyfrom said metal cathode electrode to a first level of said PN junction,with the portion of said base zone forming a PN junction with saidcentral portion of said emitter zone having a first selected dopingconcentration and the portion of said base zone forming a PN junctionwith said peripheral portion of said emitter zone having a selectedsecond doping concentration, said second doping concentration beinggreater than said first selected doping concentration, said centralportion extending into said body from said metal cathode electrode to asecond level of said PN junction, said second level lying beyond saidfirst level to form a semiconductor switching device which switches froma non conducting to a conducting state when a voltage having a selectedchange in amplitude with time is coupled to said two terminals such thatthe turn on characteristic of said emitter zone is substantiallyuniform.
 2. The device of claim 1 wherein said body has a cavity formedby cylindrical shaped walls extending from a major surface to a recessedsurface at a predetermined depth in said body, said recessed surfacebeing adjacent to said central portion of said emitter zone, and saidmetal cathode electrode makes contact with said central portion of saidemitter zone at said recessed surface and with said peripheral portionof said emitter zone at said major surface.
 3. The device of claim 2wherein said metal cathode electrode makes contact with a surfaceportion of said base zone surrounding said emitter zone, and amultiplicity of shunts of P-type semiconductivity extend from said basezone to said metal cathode electrode through said emitter zone, saidshunts being spaced at regular intervals throughout said emitter zone.4. The device of claim 2 wherein said recessed surface preferably liesat a predetermined depth from said major surface of about 15 microns.